The control of highly flexible aircraft has been a major multidisciplinary problem due to the complex fluid structure interaction (FSI) of the aircraft structure and surrounding airstream. FSI in aircraft introduces aeroelastic phenomena such as flutter from the right combination and phasing of structure vibrational modes of an aircraft wing, fuselage, empennage or other structural components. Aerodynamic forces generated by the interaction of the airstream with the flexible aircraft structure can result in an unstable oscillatory aeroelastic deformation of the structure called flutter. Aeroelastic phenomena depend on numerous structural factors (mass, stiffness, shape of the structure, particular operating conditions of the structure, etc.) and flow factors (velocity, density, turbulence, etc.).
Aeroelastic phenomena may involve both bending and torsional types of motion. In the normal operating envelope of the aircraft, the aeroelastic deformations may be relatively mild and stable. However, in certain cases of flutter, an unstable mode may result when the torsional mode extracts energy from the airstream and drives the bending mode to increasingly higher amplitudes potentially leading to catastrophic structural failure.
The avoidance of unstable aeroelastic conditions such as flutter and the determination of the maximum allowable flight parameters before flutter is encountered are critical priorities for designers of aeroelastic structures and aerospace vehicles. Exhaustive flight and wind tunnel tests are usually conducted to record and observe the flutter characteristics of the various aeroelastic structures of an aircraft over the entire operating envelope of the aircraft. Every time the external aircraft structure is modified, e.g. attachment of external stores for military combat aircraft, aircraft operation in the prescribed flight envelope must be validated with regard to flutter.
A major problem with highly flexible aircraft is the necessity for making design compromises in the wings for effectiveness over the flight envelope. Aerodynamic requirements for different flight conditions vary. Compromises may lead to stiffening of wings increasing the aircraft weight. Effective control of aircraft aeroelasticity increases the design space for aircraft substantially increasing the potential for performance and safety improvements.
In recent years, several active control strategies have been employed to control adverse aeroelastic phenomena using a variety of surface flow and structural actuators, flow and structural sensors and control strategies. Surface flow actuators include typical control surface deflections, flow displacement, injection, suction and other local flow actuation mechanisms. Structural actuators include actuators that modify the mass, stiffness, shape, and particular operating conditions of the structure, e.g. piezoelectric actuators to induce strain to strengthen a wing structure.
To effectively utilize the flow and structural actuators in a control scheme, the aerodynamic and structural loads must be obtained in some form. Sensors for monitoring and controlling aircraft aeroelasticity consist of primarily pressure sensors for determining the aerodynamic loads and structural sensors, e.g., strain gages and accelerometers, for the structural loads and moments. It is important to realize that the aircraft structure responds to the aerodynamic forcing function and therefore inherently lags the aerodynamics. As the inertial sensors are attached to the aircraft structure, the inertial sensor response significantly lags the flow sensor. Ideally, one would like to obtain an estimate of the differential or absolute aerodynamic loads without lag. However, sensors usually have lag, due to the inherent transduction mechanism; e.g., pressure sensors obtain the surface normal forces through the structural deflection of a thin diaphragm (if there is any tubing, then there is additional pneumatic lag). In addition to the lag, for accurate monitoring and control of aerodynamic loads, sensor signal levels should preferably be relatively high and relatively noiseless under aircraft environmental conditions (pressure, temperature, density) and operating conditions (electromagnetic and radio-frequency interference, vibration, etc.). Without an accurate, low-lag sensor for the aerodynamic state, aircraft control designers must infer the aerodynamic state from the response of the structure, general aircraft state (speed and angle-of-attack) and a complex aeroelastic model relating the flow and structural states. The uncertainty is as good as the FSI model, and can be especially suspect for time-varying, unsteady aerodynamic flows.